The present invention relates to an electrophotographic copying apparatus and more particularly, to a slit exposure scanning type electrophotographic copying apparatus equipped with an improved braking device for a scanning system
Commonly, in a slit exposure scanning type electrophotographic copying apparatus, suitable clutch means provided in a driving mechanism thereof is employed for starting scanning of the scanning system, i.e. reflecting mirrors, a light source or a platform to support an original to be copied thereon. In the starting as described above, however, the scanning system is subjected to abrupt start of the movement thereof with a considerable shock, and consequently to large speed variations as shown by the curve A in FIG. 7, thus resulting in indefiniteness or blur in the copied images. Therefore, it is necessary to effect a considerable amount of preliminary displacement at the starting of scanning of the scanning system. However the above arrangement has the disadvantage that an extra space equivalent to the preliminary displacement is required, with consequent reduction of copying speed, especially in a high speed copying apparatus.
In order to overcome the disadvantage as described above, there have conventionally been proposed arrangements wherein the clutch means is replaced by an exclusive motor having superior speed increase characteristics or wherein a brake means is employed which is actuated during the scanning for reduction of the variations in speed. However, the former arrangement as described above, in which the time of increase of the speed of the exclusive motor is slow as shown by the curve B in FIG. 7, is not necessarily best suited for high speed copying. Meanwhile, the latter arrangement may be classified into two types, and in one type wherein the brake means is temporarily actuated only during the scanning starting time, torque variation is undesirably brought about in the case of the high speed copying, giving rise to a possibility that the vibration of reflecting mirrors takes place upon releasing of the brake means, while in the other type in which a predetermined braking force is constantly applied during scanning of the scanning system, the necessity for driving at high torque at all times results in increase of loss of time, for example, during change-over from the scanning to returning.
On the other hand, in the known slit exposure scanning type electrophotographic copying apparatuses as described in the foregoing, the structure is generally so arranged that, in the equal size copying at a magnification of 1:1, the scanning speed of the scanning system (optical system or original platform) is equal to the moving speed of the photoreceptor surface, while in the copying at a magnification of m, the scanning speed is set to be 1/m, with the moving speed of the photoreceptor surface being made constant. For the purpose as described above, a speed change device capable of outputting revolutions corresponding to the copying magnifications is incorporated in the scanning type driving mechanism, but the speed change device as described above has the disadvantage that it has a complicated structure for an increase of the copying magnification, with consequent large size of the copying apparatus itself. Although a known stepless speed change device could be used, such a stepless speed change device is not only expensive, but too complicated, thus not being preferable from the viewpoint of actual use.